This invention relates to an apparatus for measuring the pre-tension of a threaded bolt serving, for example for closing a reactor pressure vessel. In such an apparatus a tensioning bolt is turned by a superposed automatic tightening unit in which the threaded bolt is elongated by means of a tensioning unit. The tensioning unit contains a measuring rod introduced into a central hole in the threaded bolt and in the tensioning bolt, as well as a length measuring device which determines the length difference between the threaded bolt, including the tensioning bolt, if applicable, and the measuring rod.
Such a measuring apparatus is described in U.S. Pat. No. 3,837,694. There, both the threaded bolt used for closing reactor barrels and pressure vessels and the tensioning bolt screwed into it to apply the pre-tension contain central bores forming a passage which extends to the head of the threaded bolt. A measuring rod is introduced into this hole and is screwed in at its lowest point. When the threaded bolt is pre-tensioned by means of the tensioning bolt, the measuring rod in the hole remains unstressed and the pre-tension of the threaded bolt is determined by measuring the difference in length between the inserted measuring rod and the threaded bolt, including the tensioning bolt, by means of a length measuring device, such as a dial indicator.
In the above apparatus, the measuring rods used for measuring the pre-tension are always inserted by hand into the holes of the threaded bolts with tensioning bolts and are then screwed in tight, an operation which requires a relatively long time, especially since they must be removed again after the measurement. It is an object of the present invention to utilize the rotary motion required for screwing the tensioning rod into and out of the threaded bolt at the same time for inserting the measuring rod into the threaded bolt, and for removing it.